FR2657414A1 - High temp. resistant reflector foil - Google Patents

Abstract

A high temp. resistant reflector foil has a highly reflective coating of a precious metal on a ceramic support. The novelty is that there is an IR-permeable stabilising layer (12) on the reflective coating (11). Pref. when gold is used as the coating (11) SiO is used as the stabilising layer (12). The gold coating (11) is bonded to the carrier (10) using SiO as adhesive (pref. 4-10nm thick). When Pt or Rh are used as the coating, Al2O3 is used as the stabilising layer (pref. 5nm thick). The precious metal coating is of thickness 100-500 (300) nm.

Description

"Reflective sheet stable at high temperature, in particular for the protection, upon return to the atmosphere, of spacecraft '
The invention relates to a reflective sheet stable at high temperature, comprising a coating with high reflectance of precious metal placed on a ceramic support.

 Reflective sheets of this type are, for example, for their use as a light insulating material against heat, for example for protection during the return to the atmosphere of spacecraft (DE 37 05 440).

 For use of this kind, it is absolutely essential for the reflective power of the precious metal layer remains unchanged despite the thermal effects, at temperatures up to 1450 C, for several hours. The transmission performed by the layers should then have a minimum value.

 The adhesion capacity of precious metals placed on ceramic supports by physical methods decreases considerably with temperature. We already observe, at temperatures well below the melting point of the precious metal, applied for only 30 minutes, a clear tendency to form conglomerates from the layer of precious metal placed on the ceramic support. In the case of a layer of gold, the entire layer of precious metal is already in the form of fine droplets, at 600 "C, on the ceramic support.

With this surface structure, insufficient reflective power is obtained at higher temperatures. In addition, the transmission powers increase by a value of up to 50%, since in the presence of a layer of precious metal which is not perfectly waterproof the permeability of the ceramic with respect to the infrared rays. -red produces all its effects.

 To increase the adhesion of the precious metal to a ceramic support, the method of using intermediate layers as an adhesion agent is known. Chromium, manganese, vanadium, iron, cobalt, nickel, alloys of these metals, bismuth, bismuth oxide, titanium, ruthenium, etc. and ruthenium oxide (DE 31 18 957 C2). The disadvantage of a large number of these layers lies in the fact that they have a clear tendency to diffuse inside the layer of precious metal, to seriously damage the adhesion of the precious metal on the ceramic support. , as well as its reflective power.

 The basic objective of the invention is to design a reflective sheet stable at high temperature of the kind described above, the reflecting power of which remains practically unchanged up to temperatures above 1000 ° C. and having a long service life.

 The objective is achieved, in accordance with the invention in that, on the precious metal coating, a stabilizing layer permeable to infrared rays is provided.

Other improvements of the invention are characterized in that
- in the case of gold coating, an SiO stabilization layer has been provided.

 - the gold coating was firmly fixed on the support by means of SiO as an adhesion agent.

 - The adhesion layer preferably has a thickness between 4 and 10 nm.

 - in the case of a platinum or rhodium coating, an A1203 stabilization layer has been provided.

 - The stabilization layer has a thickness between 4 and 60 nm, preferably about 5 nm.

 - the precious metal coating has a thickness of between 100 and 500 nm, preferably around 300 nm.

 It has been found that the stabilization layer placed on the surface of the precious metal opposes the formation of agglomerates, at high temperatures, so that the reflective power of the precious metal layer remains practically unchanged until high temperatures, and also in fact for quite a long time.

 According to a development of the invention, a SiO stabilization layer is provided for a reflective sheet with a gold surface. The addition of a layer of this type does not reveal, for many hours, at an air temperature of up to 1040 "C, no tendency to form agglomerates at the level of the gold placed on the ceramic support.

 Thanks to the stabilization layer, the thickness of the gold layer can be reduced to approximately 100 nm, which for use in space in particular has great advantages, for reasons of weight.

 A thickness of about 300 nm of gold in conjunction with a thin SiO stabilization layer of only 5 nm has been found to be optimal with the objective of maintaining the reflectivity up to temperatures of 1040 "C.

The gold layer may, with a view to improving the adhesion to the ceramic support, be provided using a known adhesion agent. According to another development of the invention, the
SiO as a bonding agent. The thickness of the adhesion layer can be between 4 and 10 nm, it will preferably be 50 nm, the layer of adhesion agent in SiO also further contributes to the stabilization of the gold coating.

 For the use of reflective sheets at higher temperatures up to and above 1400 "C, it is platinum or rhodium which constitute the coating with high reflective power. In this case, a layer of stabilization in A1203. In this case also, it is possible to reduce the thickness of the layer of the platinum or rhodium coating to approximately 100 nm, and that of the stabilization layer to 5 nm.

 The reflective sheets find various uses, in which the choice of materials used is optimized according to the use case.

 For uses at temperatures above 1000 "C, the choice of materials is very limited, and in reality must relate to materials which in addition to their temperature stability, must also resist oxidation. for that that the usable materials are reduced to the range of precious metals which are used in association with a ceramic substrate, oxidized ceramic, or an equivalent.

The invention will be developed thanks to the description which follows and to the drawings in which
- Figure 1 shows the reflective layer, directly on a ceramic support,
- Figure 2 shows the reflective layer bonded by means of an intermediate layer.

 FIGS. 1 and 2 show two embodiments, for which the reflective layer of precious metal 11 is connected either directly to the ceramic support 10, or else is linked by means of an intermediate layer 13, serving as an adhesive agent with the ceramic support. For reflective sheets formed in this way, the adhesion between the precious metal coating 11 and the substrate 10 greatly decreases under the effect of temperature. In the case of a gold layer, from 600 C, the entire precious metal layer is in the form of fine droplets on the ceramic substrate. This process could be observed very particularly with a substrate 10 made of fibers reinforced with ceramic oxide, with a fineness of 50 microns, such as, for example, Al 2 O 3, in mullite.

With this surface nature, unsatisfactory reflecting power was obtained at high temperatures. The transmission coefficients go up by 50%, since with a layer of precious metal that is not perfectly tight, the permeability of ceramic for infrared radiation produces all its effects. To counteract this effect, there is provided on the precious metal layer 11 a stabilization coating 12, permeable to infrared rays, so that the reflectivity for infrared rays 14, compared to a surface made of non-precious metal coated is not significantly decreased.

 For applications around 1000 "C, a gold layer 11 is preferably used. In this case, the coating 12 is made of SiO. Using a reflector constructed in this way, for a period of up to 18 hours , at an ambient temperature of up to 1040 "C, we could not observe the slightest formation of agglomerate in the gold, placed on the ceramic support 10. Measurements confirmed that no detectable difference in the reflective power of the gold layer compared to the non-thermally stressed layers did not appear.

Thus the infrared reflection measured after the test at 1000 "C amounted to 85%. The transmission coefficients of the ceramic sheet coated with gold are, after thermal stress, extraordinarily low (of the order of 5%), which argues in favor of an intact gold leaf placed directly on the ceramic support material.

 During the tests, it was subsequently found that the thickness of the gold layer, having a constant reflection power, can be reduced to approximately 100 nm. The thickness of the SiO stabilization layer can be of the order of 60 nm.

 The use of an adhesion layer 13, even in SiO, can contribute to the stabilization of the gold layer 11 at high temperatures. A gold layer with a thickness of 300 nm in conjunction with an adhesion and covering layer in SiO, with a respective fineness of 5 nm has proved to be the best, as regards the maintenance of the reflecting power. , up to temperatures of 1040 C.

 For use above 1040 "C, up to 1450" C, platinum or rhodium is used as the reflective layer. These precious metals have, in addition to a very low volatility, also a very low tendency to form oxides in the temperature zone used. To stabilize the precious metal layer on the ceramic support material, a layer 12 of Al 2 O 3 is provided.

Then the thickness of the coating of the platinum or rhodium layer can be reduced to approximately 100 nm, that of the stabilization coating can be reduced to 5 nm. Surprisingly, a structure comprising a precious metal coating 11 of 300 nm in connection with a thin stabilization coating 12 of 5 nm in A1203 has proved to be even excellent for maintaining the reflective characteristics. After thermal stress by exposure to an ambient temperature of up to 1450 "C, for a period of up to 18 hours, we had to observe a drop in reflection power of only about 15% vis-à-vis the surface not subjected to thermal stress. The transmission measurements gave values of around 7%.

 The coating thicknesses of each structure ultimately conform to each use case. For use in space, as many thin layers as possible are used.

 The layers 11 to 13 are preferably supplied by a vacuum metallization technique, in a device commonly used on the market, inside which, without interrupting the process, the vacuum metallization is carried out successively. adhesion, the precious metal layer and the protective layer. One can of course also use other methods known in the field of manufacturing diapers.

Claims (7)

 1 ") Reflective sheet stable at high temperature, comprising a coating with high reflecting power of precious metal, placed on a ceramic support, characterized in that, on the precious metal coating (11), a stabilization layer is provided ( 12) permeable to infrared rays (14).  2 ") Reflective sheet according to claim 1, characterized in that, in the case of gold coating (11), a stabilization layer (12) in SiO has been provided.  3 ") Reflective sheet according to claim 1 or 2, characterized in that the gold coating (11) has been firmly fixed on the support (10) by means of SiO as an adhesion agent (13).  4 ") Reflective sheet according to claim 3, characterized in that the adhesion layer (13) preferably has a thickness between 4 and 10 nm.  5 ") Reflective sheet according to claim 1, characterized in that, in the event of coating (11) of platinum or rhodium, a stabilization layer (12) of Au203 is provided.  6) Reflective sheet according to one of the preceding claims, characterized in that the stabilization layer has a thickness between 4 and 60 nm, preferably about 5 nm.  7 ") Reflective sheet according to one of the preceding claims, characterized in that the precious metal coating has a thickness of between 100 and 500 nm, preferably around 300 nm.